Cycle Phase Calculator - Calendar Estimate

Estimate your menstrual cycle phase, cycle day, next period, ovulation date, and fertile window from your last period and average cycle length.

Estimate

Required: enter the first day of your last period, average cycle length, and the date to evaluate.

First day of last period

Cycle day 1 is the first day bleeding begins.

Date to evaluate

Average cycle length

Enter your own average; 28 days is only a common example.

Typical bleeding duration

Estimated luteal-phase length

Calendar estimates often use 14 days, but the interval can vary.

Calendar estimate only—not a diagnosis or confirmation of ovulation. Do not use this result as the sole method to prevent pregnancy. Irregular cycles, hormonal contraception, pregnancy, breastfeeding, PCOS, and perimenopause can make estimates unreliable.

Estimated result

Cycle detail	Estimated date or value

Cycle Phase Calendar Estimate

Cycle day 1 is the first day of menstrual bleeding. This calculator projects repeated cycles from the entered average cycle length and estimates ovulation by subtracting the entered luteal-phase assumption from that cycle length.

Estimated ovulation day = Cycle length - Luteal length

The estimated fertile window is the six-day interval from five days before the estimated ovulation date through that date. Actual ovulation can vary substantially, even in people who describe their cycles as regular, so a calendar estimate does not confirm ovulation and should not be the sole method used to prevent pregnancy.

What is a Menstrual Cycle?

The menstrual cycle is a series of natural changes in hormone production and the structures of the uterus and ovaries of the female reproductive system that make pregnancy possible. The cycle is required for the production of oocytes, and for the preparation of the uterus for pregnancy. The length of a menstrual cycle can vary, but the average is around 28 days.

How to Calculate Your Cycle Phase?

The following steps outline how to calculate your menstrual cycle phase.

First, input the first day of your last period.
Next, input the average length of your menstrual cycle in days.
Then, input the current date.
Click the “Calculate” button to find out the current phase of your menstrual cycle.
The calculator will display the phase: Menstrual, Follicular, Ovulatory, or Luteal.

Example Problem :

Use the following variables as an example problem to test your knowledge.

First Day of Last Period = 03/01/2023

Average Cycle Length = 28 days

Current Date = 03/15/2023